1. Field of the Invention
This invention relates to a focus detecting apparatus.
2. Description of the Related Art
Sharpness detecting methods of varied kinds have been known for detecting the sharpness of the image of a photographing object formed on an image plane through a picture signal obtained from the image sensor of a video camera or the like in carrying out a focusing operation by driving an optical system in such a way as to maximize the sharpness thus detected.
In the first type of these known methods, a high frequency component of the picture signal is extracted by means of a by-pass filter and a differentiation circuit or the like; and the sharpness of the object image is evaluated by the intensity of the extracted high frequency component. In carrying out the focusing operation in accordance with this method, the direction in which the optical system is to be driven is determined by comparing the degrees of sharpness of two differently formed images obtained during the process of driving the optical system. An and an infocus state of the optical system is obtained by bringing the optical system to a stop at a point where the image sharpness is obtained in a maximum degree.
In one example of the second type of the known methods which is disclosed in Japanese Patent Publication No. SHO 54-44414, the fact that the size of the medium density range of the object to be photographed varies with the degree of focus is utilized. In other words, the medium density area becomes large in the event of defocus and small in the case of an in-focus state. The disclosed focus detecting device is thus arranged to determine an in-focus state at a point where the medium density area reaches a minimum size. Aother example of the second known method is disclosed in Japanese Patent Publication No. SHO 52-30324. In this case, unlike the device of Japanese Patent Publication No. SHO 54-44414 which is arranged to detect the size of the medium density range, the size of either the high density range or low density range of an object is detected; and, by utilizing the fact that the size of the high or low density range varies with the focusing degree, an in-focus state is judged to be attained at a point where such a density range reaches a maximum size.
Generally, however, the degree of detected image sharpness greatly varies according to the kind and contrast of the object to be photographed. Therefore, the above-stated first and second prior art methods present the following problem. Since the detected sharpness changes, for example, due to a slight movement of the object or due to a change in the illuminating conditions, an erroneous maximum value of sharpness might be obtained during a driving operation on the optical system. In that event, the optical system tends to come to a stop at a point which is not a real in-focus position. Besides, once the optical system comes to a stop in this manner, a differently formed image is no longer obtainable for further comparison. Then, the device becomes incapable of correctly detecting the actual focused state of the optical system. Further, in accordance with the conventional known methods in general, not only the above-stated first and second methods, the position of the optical system must be at least once shifted past an in-focus point. Therefore a picture thus obtained comes to blur after it has reached an in-focus state and before it comes to recover the in-focus state. As a result, the picture becomes disagreeable.